The present invention relates to a switching power supply circuit having a power factor improving function.
FIGS. 10 and 11 are circuit diagrams showing different examples of a power factor improving circuit in a switching power supply circuit. FIG. 11 shows only the section of a power factor improving circuit.
FIG. 10 shows a power factor improving circuit 20a of a capacitive coupling type using capacitor voltage division.
The power supply circuit is formed by providing a self-excited voltage resonance type switching converter with the power factor improving circuit 20a for improving the power factor.
The power supply circuit shown in the figure is provided with a bridge rectifier circuit Di for subjecting a commercial alternating-current power AC to full-wave rectification.
An output rectified by the bridge rectifier circuit Di is stored in a smoothing capacitor Ci via the power factor improving circuit 20a, whereby a rectified and smoothed voltage Ei is obtained across the smoothing capacitor Ci.
For description of the voltage resonance type converter, reference is to be made to embodiments of the present invention.
A parallel resonant capacitor Cr is connected to a collector of a switching device Q1. Capacitance of the parallel resonant capacitor Cr and leakage inductance L1 on the primary winding N1 side of an isolating converter transformer PIT form a primary-side parallel resonant circuit of the voltage resonance type converter. During the off period of the switching device Q1, a voltage across the resonant capacitor Cr practically forms a sinusoidal pulse waveform as a result of the effect of the parallel resonant circuit, and thus a voltage resonance type operation is obtained.
The power factor improving circuit 20a has a choke coil Ls and a fast recovery type diode D1 connected in series with each other and inserted between a positive output terminal of the bridge rectifier circuit Di and a positive terminal of the smoothing capacitor Ci. A filter capacitor CN is provided in parallel with the series connection circuit of the choke coil Ls and the fast recovery type diode D1, thereby forming a normal-mode low-pass filter in conjunction with the choke coil Ls.
A parallel resonant capacitor C10 is provided in parallel with the fast recovery type diode D1. The parallel resonant capacitor C10 forms a series resonant circuit in conjunction with the choke coil Ls. The series resonant circuit thereby has an effect of controlling increase in the rectified and smoothed voltage Ei at light load.
The parallel resonant capacitor Cr is connected to the power factor improving circuit 20a at a node that connects the choke coil Ls, an anode of the fast recovery type diode D1, and the parallel resonant capacitor C10 with each other, so that a switching output obtained in the primary-side parallel resonant circuit is fed back to the power factor improving circuit 20a. 
Thus, with the configuration of the power factor improving circuit 20a shown in the figure, the switching output obtained in the primary-side parallel resonant circuit is fed back to the rectified current path via the capacitive coupling of the parallel resonant capacitor Cr.
Since the parallel resonant capacitor Cr is connected to the anode of the fast recovery type diode D1 in the power factor improving circuit 20a, the parallel resonant capacitor Cr and the parallel resonant capacitor C10 are in a state of being connected in series with each other. Specifically, a voltage resonance pulse voltage appearing as a voltage across the parallel resonant capacitor Cr is divided by a capacitance ratio between the parallel resonant capacitor Cr and the parallel resonant capacitor C10. The voltage is fed back to the smoothing capacitor Ci via the parallel resonant capacitor C10 connected in parallel with the fast recovery type diode D1, and thus a circuit system of a voltage feedback type is formed.
This circuit configuration divides a primary-side voltage resonance-pulse voltage Vcp=600 V, for example, into voltages in a ratio of about 3:1 by means of the primary-side parallel resonant capacitors Cr and C10, and then feeds back a high-frequency sinusoidal pulse voltage of 150 V.
At times near a positive and a negative peak of an alternating-current input voltage VAC, the fast recovery type diode D1 conducts, and the smoothing capacitor Ci is charged with a steep pulse charging current from the alternating-current input power supply AC.
At other than the times near the positive and negative peaks of the alternating-current input voltage VAC, the fast recovery type diode D1 is allowed to repeat switching operation by the pulse voltage being fed back. During the off period of the fast recovery type diode D1, a parallel resonance current caused by the parallel resonant capacitor Cr, the inductance LS, and the capacitor CN flows. During the on period of the fast recovery type diode D1, a high-frequency charging current flows from the alternating-current input power supply AC to the smoothing capacitor Ci via the inductance LS.
This operation increases the conduction angle of an alternating input current IAC, thereby making it possible to improve the power factor.
FIG. 11 shows a power factor improving circuit 20b of a diode coupling type using a tertiary winding system.
The power factor improving circuit 20b has a choke coil LS and a Schottky diode D1s connected in series with each other and inserted between the positive output terminal of the bridge rectifier circuit Di and the positive terminal of the smoothing capacitor Ci.
A filter capacitor CN is inserted in parallel with the series connection of the choke coil LS and the Schottky diode D1s, thereby forming a normal-mode low-pass filter in conjunction with the choke coil LS.
A tertiary winding N3 of an isolating converter transformer PIT is connected via a series resonant capacitor C3 to a node that connects an anode of the Schottky diode D1s and the choke coil LS with each other, whereby the switching output voltage obtained in the primary-side parallel resonant circuit is fed back to the power factor improving circuit 20b. 
In this case, around peaks of the absolute value of the alternating-current input voltage VAC, the Schottky diode D1s conducts, and a charging current I1 flows from the alternating-current input power supply AC to the smoothing capacitor Ci via the choke coil LS and the Schottky diode D1s. At the same time, a voltage resonance pulse voltage of the tertiary winding N3 is fed back to a series circuit of the series resonant capacitor C3 and the Schottky diode D1s for switching operation of the Schottky diode D1s. Thereby, a flowing range of the alternating input current IAC is extended, and thus the power factor is improved.
When the absolute value of the alternating-current input voltage VAC is lowered, the Schottky diode D1s becomes nonconductive, and the voltage resonance pulse voltage of the tertiary winding N3 is turned into a series resonance voltage by a series circuit of the series resonant capacitor C3, the choke coil LS, and the filter capacitor CN.
The two circuit examples are shown above, and the configuration of FIG. 11 has the higher AC/DC power conversion efficiency xcex7AC/DC. Characteristics of the AC/DC power conversion efficiency xcex7AC/DC and the power factor PF in this case are shown in FIGS. 12 and 13.
FIG. 12 shows characteristics of the power factor PF and the AC/DC power conversion efficiency xcex7AC/DC when the load power Po is varied from 40 W to 200 W. FIG. 13 shows characteristics of variations in the power factor PF and the AC/DC power conversion efficiency xcex7AC/DC when the alternating-current input voltage VAC is varied from 80 V to 260 V.
As is understood from the figures, it is possible to maintain a power factor PF of 0.7 or more and achieve an AC/DC power conversion efficiency xcex7AC/DC of 90% or more over wide ranges of the load power and the alternating-current input voltage.
However, a switching power supply circuit having the prior art power factor improving circuit 20a or 20b as described above has the following problems.
First, increase of the amount of voltage feedback to the power factor improving circuit 20 in a state of a maximum load power for improvement of the power factor to 0.8 or more extends a load power region and an alternating-current input voltage region where zero volt switching operation, a condition for stable operation of the primary-side voltage resonance converter, cannot be performed. Therefore, the power factor cannot be improved to 0.8 or more.
FIGS. 14A to 14J show operating waveforms of parts of the circuit example shown in FIG. 11.
Around the positive and negative peaks of the alternating-current input voltage VAC, a series resonance current IC3 of the tertiary winding N3 and the series resonant capacitor C3 is superimposed on a current ID1 flowing from the inductance Ls and the Schottky diode D1s. Thus, an excessive charging current as shown in FIG. 14I flows as a current I1 to the smoothing capacitor Ci.
Therefore, increase of the number of turns of the tertiary winding N3 for a larger amount of voltage feedback for improvement of the power factor PF narrows a load power region and an alternating-current input voltage region where zero volt switching operation, a condition for stable operation of the switching device Q1 of the primary-side voltage resonance converter, can be performed. This renders the zero volt switching operation unstable with variation in the load power Po and the alternating-current input voltage VAC. Thus, the power factor cannot be improved to 0.8 or more.
In addition, the AC/DC power conversion efficiency xcex7AC/DC cannot be increased further in the state of the maximum load power.
An object of the present invention is to provide a switching power supply circuit having a power factor improving function, which is capable of performing a zero volt switching operation without narrowing the range thereof even when the power factor is improved.
To achieve the above object, according to the present invention, there is provided a switching power supply circuit, including: a smoothing means having two smoothing capacitors connected in series with each other for smoothing a rectified current and thereby outputting a double direct-current input voltage; an isolating converter transformer for transmitting an output on a primary side thereof to a secondary side thereof, the isolating converter transformer including a gap formed so as to provide loose coupling at a desired coupling coefficient; a switching means including a switching device for interrupting the double direct-current input voltage and outputting the interrupted voltage to a primary winding of the isolating converter transformer; a primary-side resonant circuit formed by at least a leakage inductance component of the primary winding of the isolating converter transformer and capacitance of a primary-side parallel resonant capacitor for converting operation of the switching means into voltage resonance type operation; a power factor improving rectifier means for rectifying an alternating-current power and thereby supplying the rectified current to the smoothing means and for improving a power factor by interrupting the rectified current according to a switching output voltage obtained in the primary-side resonant circuit and fed back to the power factor improving rectifier means via a series resonant capacitor and a tertiary winding formed by winding a wire of the primary winding of the isolating converter transformer; a secondary-side resonant circuit formed on the secondary side by a leakage inductance component of a secondary winding of the isolating converter transformer and capacitance of a secondary-side resonant capacitor; a direct-current output voltage generating means for rectifying an input alternating voltage obtained in the secondary winding of the isolating converter transformer and thereby generating a secondary-side direct-current output voltage, the direct-current output voltage generating means including the secondary-side resonant circuit; and a constant-voltage control means for effecting constant-voltage control of the secondary-side direct-current output voltage according to level of the secondary-side direct-current output voltage.
The power factor improving rectifier means has a first rectifier circuit formed by two fast recovery type diodes connected in series with each other and a second rectifier circuit formed by two slow recovery type diodes connected in series with each other. The tertiary winding is connected via the series resonant capacitor to a node that connects the two fast recovery type diodes with each other, whereby the switching output voltage is fed back to the power factor improving rectifier means. Each of the two fast recovery type diodes interrupts the rectified current according to the fed-back switching output voltage, whereby the power factor is improved.
Alternatively, the power factor improving rectifier means may have a rectifier circuit formed by two fast recovery type diodes connected in series with each other. The tertiary winding is connected via the series resonant capacitor to a node that connects the two fast recovery type diodes with each other, whereby the switching output voltage is fed back to the power factor improving rectifier means. Each of the two fast recovery type diodes interrupts the rectified current according to the fed-back switching output voltage, whereby the power factor is improved.
With this configuration, the voltage resonance pulse voltage generated in the primary-side voltage resonance converter is fed back to the power factor improving rectifier means via the tertiary winding and the series resonant capacitor by magnetic coupling. Thereby, the flowing range of the alternating input current IAC is extended, and thus the power factor can be improved to about 0.9, for example.
In addition, by subjecting the alternating-current input to voltage doubler rectifier operation, it is possible to improve AC/DC conversion efficiency and reduce a ripple component of the direct-current output voltage. Also, the first rectifier circuit and the second rectifier circuit shunt the current to be stored in the smoothing means. Thus, it is possible to secure a zero volt switching operation range even when the power factor is improved.